1. Field of the Invention
The present invention relates to an apparatus and method for selecting a correlation scheme based on a carrier frequency error; and, more particularly, to a correlation scheme selecting apparatus and method which acquires initial synchronization efficiently by calculating frequency-error criterion threshold between correlation schemes and selecting a correlation scheme for a corresponding region.
This work was supported by the IT R&D program of MIC/IITA [2007-S-008-01, “Development of 21 GHz Band Satellite Broadcasting Transmission Technology”].
2. Description of Related Art
Recently, as the distinction between broadcasting and communications becomes vague and bi-directional services become available, there are demands for broadband services and reliable transmission services. Thus, satellite communication systems employ Adaptive Coding and Modulation (ACM) to acquire a high transmission capacity with a given bandwidth and signal power.
Particularly, Digital Video Broadcasting satellite Second Generation (DVB-S2), which is the standard of European Digital Video Satellite Broadcasts, uses the ACM scheme that determines an optimum modulation scheme and an optimum coding rate according to a channel condition to thereby maximize the transmission efficiency.
Due to the use of the ACM scheme, each receiver of a satellite communication system which provides Ka-band services ranging from 18 to 31 GHz can secure a transmission capacity up to 100 to 200% by adaptively controlling a data link according to propagation conditions. However, it requires a process for detecting a varying structure of transmission frame as well as frame sync in order to recover a carrier.
Meanwhile, the DVB-S2 system performs correlation calculation in order to acquire initial sync under an environment where carrier frequency errors exist. The frequency errors existing between a transmitter and a receiver degrade the performance of a correlator.
Accordingly, a differential correlation between a received signal and a Start of Frame (SoF) which is a preamble obtained during a previous frame sync process is used to minimize the influence caused by the frequency error.
However, under the environment where there is no frequency error or frequency errors are present below a certain level, a case employing the differential correlation causes severer performance degradation than a case employing coherent correlation. Therefore, it is more advantageous to adaptively select and use either the coherent correlation or the differential correlation scheme according to a change in the level of the frequency error, than to use each one of the correlation schemes.